Wheel aligner measurement module attachment system

ABSTRACT

In a vehicle alignment system an optical sensing mechanism is structurally coupled to a vehicle supporting lift for movement in unison with the lift so that the field of view of the sensing mechanism encompasses a wheel of a vehicle positioned on the lift during lift movement. Sensing modules may be selectively placed so that they are in correspondence with the vehicle wheel base. Modules may be deployed to extend outwardly from the lift, for viewing the wheel during an alignment procedure, or retract to a adjacent the lift for stowing. A protective cover extends above the sensing modules in the retracted position.

RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application No. 60/680,497 filed on May 13, 2005,which is incorporated by reference herein its entirety.

TECHNICAL FIELD

The present disclosure relates to visual wheel alignment systems, moreparticularly to the positioning of optical sensors for unobstructedfield of view of vehicle wheels.

BACKGROUND

Known systems for alignment of wheels of motor vehicles employcomputer-aided, three-dimensional machine vision alignment apparatus.Examples of so-called “3D alignment” systems are described in commonlyassigned U.S. Pat. No. 5,724,743 to Jackson, entitled “Method andapparatus for determining the alignment of motor vehicle wheels,” andcommonly assigned U.S. Pat. No. 5,535,522 to Jackson, entitled “Methodand apparatus for determining the alignment of motor vehicle wheels,”both of which are commonly assigned to the assignee of the presentapplication and incorporated herein for reference in their entireties.Sensors, such as cameras, view alignment targets affixed to the wheelsin a known positional relationship to determine the position of thealignment targets relative to the alignment cameras. The alignmentcameras capture images of the targets. From these images, the spatialorientation of the wheels can be determined and related to specificationalignment characteristics. Wheel alignment adjustments can be made,while maintaining camera sensing, until the captured images correspondto specification values.

More recent versions of 3D alignment systems favor using identifiablewheel features to determine the spatial orientation of a wheel, in lieuof attaching targets to the wheels. With such operation, the cost oftargets, a variety of attachment devices needed for different vehiclemodels, and the technician labor involved in the proper attachment ofthe targets, is eliminated. A wheel characteristic that can be sensed toderive the necessary spatial relationships, for example, may be theboundary between tire and wheel, or simple targets attached to thewheel.

These machine vision wheel alignment systems utilize measurement modulesthat need an unobstructed side view of the vehicle wheels. Computersoftware processes the images to distinguish the wheel from the tire andcalculate wheel alignment parameters based on the extracted image of thewheel. The measurement modules typically include one or more digitalcameras, illumination devices for illuminating the wheel during imagecapture, and a structure for supporting these various devices. A typicalwheel alignment system may have one measurement module for each wheel ofthe vehicle being measured. To optimize measurement performance thedistance from the wheel to the measurement module must be predeterminedand allowed to vary over a fairly narrow range. It is advantageous tohave the measurement module in a position such that it is longitudinallyaligned with the center of the wheel it is measuring.

Systems that have been developed to date typically use self-standingmeasurement modules that contain the actual measuring devices. Themeasurement modules are installed at a fixed height to the side of thevehicle lift and can be in the way of the shop personnel when they arenot being used for a wheel alignment. During a wheel alignmentmeasurement, the vehicle is usually placed on a vehicle lift. Normallythe operator will raise or lower the lift during the alignment processto make adjustment of the vehicle easier. The cameras require very wideangle lenses to be able to capture wheel images for vehicles withvarious wheelbases and for vehicles that are moved relative tostationary measurement modules. The wheel alignment process also mayrequire sensing while the vehicle is moved longitudinally when the liftis in the lowered position.

SUMMARY OF THE DISCLOSURE

The subject matter described herein overcomes these short comings. Anoptical sensing mechanism is structurally coupled to a supporting liftfor movement in unison with the lift so that the field of view of thesensing mechanism encompasses a wheel of a vehicle positioned on thelift during lift movement. Preferably, the optical sensing mechanismincludes a sensing module in which a pair of sensors, which may becameras, are positioned at a fixed distance from each other. Thedistance is set to place the pair of sensors along a longitudinaldirection of the lift approximately at opposite ends of the wheeldiameter so that the field of view of each sensor encompasses asubstantial portion, or the entire side, of the wheel. Alternatively, asingle sensor may be utilized.

A mounting member may be attached to each side of the lift. A pluralityof positions are provided in each mounting member for securely mountinga pair of the sensing modules to the member and, thus, to the lift. Themounting member positions are located such that one or both of thesensing modules may be selectively placed so that both modules are incorrespondence with the vehicle wheel base.

A deployment mechanism is coupled between the lift and the sensingmodule. In an extended position of the deployment mechanism, the sensingmodule extends outwardly from the lift for viewing the wheel during analignment procedure. In a retracted position of the deployment mechanismthe sensing module is adjacent the lift for stowing when there is noalignment procedure taking place. The retracted position is displaced inthe longitudinal direction of the lift from the extended position. Aprotective cover is attachable to the lift to extend above the sensingmodule when it is in the retracted position. The deployment mechanismcan be locked in each of the extended position and retracted positionfor stabilizing the sensing module.

A separate protective cover may be provided for each sensing module.Alternatively, a single protective cover can be provided to extend overthe pair of sensing modules on each side of the lift. In the lattercase, a mounting member position is provided to position one of the pairof sensing modules such that the pair of sensing modules are adjacent toeach other in their retracted positions.

Additional advantages will become readily apparent to those skilled inthis art from the following detailed description, wherein only thepreferred embodiments are shown and described, simply by way ofillustration of the best mode contemplated of carrying out theinvention. As will be realized, the invention is capable of other anddifferent embodiments, and its several details are capable ofmodifications in various obvious respects, all without departing fromthe invention. Accordingly, the drawings and description are to beregarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

Implementations of the present invention are illustrated by way ofexample, and not by way of limitation, in the figures of theaccompanying drawings and in which like reference numerals refer tosimilar elements.

FIG. 1 is a perspective view of an aligner module attachment system in alowered position in accordance with the present invention.

FIG. 2 is a partial perspective view of a sensing module of the systemof FIG. 1 deployed in an extended position.

FIG. 3 is a partial perspective view of a sensor configured in thesystem of FIG. 1.

FIG. 4 is a side view of the system of FIG. 1 in a raised position.

FIG. 5 is a partial perspective view of a variation of the alignermodule attachment system of FIG. 1.

FIG. 6 is a partial perspective view of a pair of sensing modules of thesystem of FIG. 1 in retracted position.

FIG. 7 is a schematic diagram of the system of FIG. 1 includingadditional features.

FIG. 8 is a partial perspective view of a variation of the alignermodule attachment system of FIGS. 1-6.

FIG. 9 is a block diagram for controlling deployment mechanismoperation.

FIG. 10 is a partial perspective view of a turntable positioningapparatus in accordance with the present invention.

FIG. 11 is a partial perspective view of another embodiment inaccordance with the present invention.

FIG. 12 is a partial perspective view of another embodiment inaccordance with the present invention.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of an exemplary module attachment system.Vehicle 10 is schematically represented with front wheels 12 and rearwheels 14 supported by lift 16. The front wheels rest on turntables 17,the back wheels on skid plates 19. Structurally coupled to runways 18 ofthe lift are sensing modules 20 at each wheel. Protective cover 21 ismounted to the lift between modules 20. Sensing module 20 and adeployment mechanism that couples it to the lift are shown in moredetail in the partial perspective view of FIG. 2. The sensing module 20comprises a pair of sensors, or pods, 22 held at a fixed distance by bar24. Each sensor is partially surrounded by a protective guard cage 26.The module is joined to a deployment mechanism comprising struts 28 and30 and beam member 32. Beam member 32 may be affixed to the lift runway18 by bolts or other suitable means.

As shown in FIGS. 1 and 2, the sensing module 20 is in a position thatextends outwardly from the lift. The distance between the pair ofsensors preferably is substantially equal to the diameter of an averagevehicle wheel. The beam member 32, struts 28 and 30 and bar 24 areconfigured as a rectangle, wherein bar 24 is substantially parallel tothe longitudinal direction of the lift. In this position, the wheel isin the field of view of both sensors and an alignment procedure can takeplace. Each of the four corners of the deployment mechanism contains apivot element, not shown. A ball detent or equivalent means may beincluded to set the deployment mechanism at the rectangularconfiguration shown. Locking knob 34 can be used to hold the setting.

Sensor 22 is shown in more detail in FIG. 3. Housing 36 includes acamera having a lens 38 surrounded by illumination devices 40. An arrayof LEDs direct radiation, which may be infrared, to the wheels. Thecamera, via lens 38, captures a digital image of the tire and wheelcombination. In a well known manner, computer software can process theimages obtained from both sensor cameras to distinguish the wheel fromthe tire and calculate wheel alignment parameters based on the extractedimage of the wheel. With a sensing module mounted to the lift at each ofthe vehicle wheels, image input is obtained to permit all necessaryalignment adjustments.

FIG. 4 is a side view of the system of FIG. 1 with the lift raised. Bycoupling the sensing modules directly to the lift they automaticallymove up and down with the vehicle. The wheel is kept within the field ofview of the sensors. The spatial orientation of the wheel with respectto the sensors is maintained as the height of the lift is varied.

The necessity of very wide angle lenses can be avoided by centering thesensing modules longitudinally on the wheels as closely as possible. Thearrangement illustrated in FIGS. 1 and 4 provide this objective. FIG. 5is a partial perspective view of a variation of the aligner moduleattachment system to accommodate for a wide dimensional range of vehiclewheelbase dimensions. A rail 42 is attached to each side of the lift.Hanging clip elements 44 are engaged within a slot running the length ofrail 42. Knobs 46 in a loosened state permit the clip elements to slidelongitudinally along the rail 42. When knobs 46 are tightened, the clipsare held in position on the rail. Of course, other means for locking theclips may be employed. Beam member 32, which is not affixed to the liftrunway as in FIG. 1, is secured to the clip elements 44. The sensingmodule is moved in concert with the clips along the rail to obtain thespatial orientation of the wheel with respect to the sensors illustratedin FIG. 1. The modules for the forward and rear wheels can thus be setto be in correspondence with any vehicle wheel base.

Slots 48 in the rail 42 provide a guide to the positioning of the clips.Rail 42 may extend along the lift a length sufficient to accommodateboth front and rear wheel sensing modules. The longitudinal position ofboth modules can thus be adjusted. Alternatively, one of the sensingmodules can be mounted to be stationary as shown in FIG. 1, and the rail42 can extend a length sufficient to adjust the position of the othersensing module appropriately to the dimension of any vehicle wheelbase.

The locking knob 34, shown in FIG. 2, can be loosened so that thedeployment mechanism can pivot about the four corners to assume aparallelogram configuration. When not in use, the mechanism can bepivoted to the retracted position shown in FIG. 6. In the retractedposition, sensors 22 and connecting bar 24 are stowed under protectivecover 21. The locking knob can then be tightened to maintain themechanism in the retracted position. As illustrated, in the retractedposition both modules are side-by-side under the cover 21. To attainthese positions, the left hand deployment mechanism has pivoted in acounterclockwise direction and the right hand deployment mechanism haspivoted in the clockwise direction. The right hand module has beenrepositioned along rail 42 for a parked deployment alongside the lefthand module.

The system of FIG. 1 may be provided with additional features asillustrated in the schematic diagram of FIG. 7. Built into each moduleis a two-axis gravity gauge 50 and a pair of toe sensors 52. The toesensors 52 reference each of the modules with respect to each other(angularly) in the toe plane (horizontal plane). The toe sensor can be alinear camera and at least one LED or laser light source on each module.Alternatively the toe sensors can be replaced with a 3D measurementapparatus such as used in the previously referenced three-dimensionalmachine vision alignment apparatus and consisting of a camera containinga 2 dimensional sensor array attached to one pod that measures thethree-dimensional position of a known target attached to another pod.

If all of the cameras in a given system are rigidly fixed relative toEarth, the system can be calibrated with a gravity referenced target sothat each camera knows the direction of gravity relative to itself. Whenthe cameras are calibrated in this manner they can measure angles of thewheels relative to gravity. If any camera moves relative to Earth, inthe absence of a gravity gauge, it must be recalibrated.

With the use of the gravity gauges, a relative position calibrationbetween the camera and the attached gravity gauge in a measurementmodule need only be performed one time. This calibration teaches thecamera its orientation relative to a vertical gravity vector measured bythe gravity gauge. During normal operation the gravity gauge value ismeasured continuously and the gravity measurement is used to correct themeasurements made by the camera system for changes in the orientation ofthe camera relative to the calibrated zero gravity vector of the camera.The calibration of the camera/gravity gauge assembly can be done in thefactory at the time of assembly for system configurations that havemeasurement modules rigidly connected to a gravity gauge, oralternatively it could be done on site in the wheel alignment shop. Asthe gravity gauges provide a continuous gravity reference measurementavailable for the system, there is no need to manually recalibrate themeasurement module if the camera moves relative to Earth. Camber valuescan be determined relative to this gravity reference. If the measurementmodule moves, the change in its orientation to the gravity vector ismeasured and the calculated camber values are adjusted accordingly.

The gravity gauges can also be used to detect vibration in themeasurement modules. For example, after the position of a measurementmodule is adjusted in a movable measurement module system, it may bedesirable to monitor the stability of the gravity gauge readings to makesure the module is stable before having the camera collect images. Asthe gravity gauges have much faster response time than the cameras, theyprovide a better stability indicator. Secondly, if vibration is detectedin an image, the gravity gauge can be used to determine if the vibrationis due to movement of the measurement module or the object. It may alsobe possible to determine the best time to take a camera reading based onthe vibration signal obtained from the gravity gauge. By knowing how thecamera is moving it could be enabled to make a measurement at the momentthat its position is the most stable.

By attaching the sensing modules directly to the lift they automaticallymove up and down with the vehicle, keeping the wheel within the field ofview of the sensors. The sensing modules may be positioned on the liftto correspond with any vehicle wheel base and provide an unobstructedview of the wheels. Attachment of the modules to the lift avoids theobstructions caused by conventional self-standing systems. Theretraction mechanism allows the modules to be stowed in a protectedspace, when not in use.

In a variation of the structure depicted in FIGS. 1-6, positioning ofthe modules may be performed automatically, as depicted in FIG. 8. Motor60 is coupled to the pivot element at one of the four corners of thedeployment mechanism. Track 62 is attached to the side of the lift rack.Pinion 64, attached to the deployment mechanism, is engaged with track62. Pinion 64 is coupled to stepper motor 66.

FIG. 9 is a block diagram for controlling deployment mechanismoperation. Coupled to CPU processing unit 70 are motor controller 72,camera module 74 and encoder 76. While shown only schematically, cameramodule 74 represents all sensors in the module pods that provide sensedimage information to the CPU 70. Encoder 76 may be coupled to motor 66to translate rotational movement thereof to linear travel along thetrack 62. The encoded information is output to the CPU 70. The CPUoutputs signals to the motor controller 72 for controlling motor 66.Limit switches 78 located at each end of the range of permissible moduletravel are coupled to the motor controller. The schematic representationof motor block 66 may also represent motor 60 which, under the controlof the motor controller, is activated to pivot the deployment mechanismto its open and closed positions. The extent of travel in each of thesepositions can be controlled by limit switches 78.

In operation, a technician starts a new alignment with a user input tothe CPU 70. The four modules are moved out of their protective cover toextend outward of the lift rack, for example, approximately two feet, bymeans of motor 60. The mechanism is then driven by stepper motor 66along the lift rack to a pre-defined location. In most cases, thepre-defined location is at the rearward position of travel on the trackand within a predictable wheel location window. The positionalrelationship between the sensors and the lift rack are calibrated priorto alignment sensing. The technician drives the vehicle onto the liftrack. As the wheel passes in front of its sensing module, the camera(s)take images of the wheel. The wheel is tracked by moving the deploymentmechanism forward until it is properly oriented with respect to thewheel. As an alternative to taking images of the wheel for this purpose,measurements may be made by sensing markers or targets placed on thewheels. These and other alternatives are discussed below.

The modules may include one or more sensors to measure distance betweenfront and rear mechanisms. For example, two LED light sources may beseparated by a known amount in one module and a linear optical sensorplaced in the other module. By measurement of the sensed separation oflights output by the LED lights and the use of triangulation, thedistance between modules can be determined. Yet another way to determinemodule position is to convert rotational steps of the stepper motor 66,attached to the deployment mechanism, to linear motion with respect to ahome position.

Instead of trying to find the wheel as the vehicle is driven onto thelift, the technician may enter data that identifies the type of vehiclethat is to be measured. A database can be constructed that containspreferred alignment numbers with specifications obtained from the OEMsas well as wheelbase information stored for the vehicles. The deploymentmechanisms can then be moved to their correct position by reading anencoder attached to the rack.

After measurements of the wheel locations with respect to the pods havebeen obtained, and the relationship between the pods with respect to thelift rack has been calibrated, the turntables are positioned in or outto place them at the correct width for the vehicle to be measured. FIG.10 is a partial perspective view of a turntable positioning apparatusthat may be implemented for this purpose. Front wheel 12 is shownpositioned on turntable 17 after the turntable has been properlypositioned. Motor 80 and pinion 82 are attached to the turntable 17.Track 84 is mounted on a beam of the lift rack that is perpendicular tothe longitudinal direction. The turntable is driven in thisperpendicular direction by interaction between the track 84 and thepinion 82, which is driven by motor 80 under controller of CPU 70 andmotor controller 72, as illustrated in FIG. 9.

If the wheel cannot be properly sensed when the car is driven by thecameras, or measurement is to be made of a vehicle that is already onthe lift rack, a search procedure can be undertaken. Several sweeps ofmultiple images of the wheel (or wheel markings, target) at differentillumination levels are made to ensure that proper sensing illuminationis achieved. The sweeps are implemented by activation of motor 66 inresponse controller 72, CPU 70, and limit switches 78. Illuminationlevels can be set empirically, depending on camera sensitivity, dynamicrange, light output and reflectance or absorption of light by the objectbeing observed. After a sufficient number of sweeps, the deploymentmechanism is moved a pre-defined distance and the sweep illuminationprocedure is repeated. The distance moved should be smaller then thefield of view of the camera to ensure that movement does not miss theobject. This procedure continues until the wheel image has been acquiredor if the end of travel has been reached. In the latter case, directionis reversed and the sweep procedure continues. Once the desired objecthas been acquired, measurement can be made of the distance that thewheel is offset from ideal location in the camera's field of view andthe deployment mechanism is moved in the appropriate amount anddirection for tracking.

Wheel offset with respect to the cameras may be determined uponobtaining an image of the wheel rim by one of the cameras. As the wheelrim is circular and the approximate radius is known, the angle at whichthe circle is foreshortened is measured. This angle is then converted tooffset distance. With the use of the second camera of the module, therim can be identified in foreshortened images by both cameras. Atriangulation on the wheel features provides a further “fit” to optimizethe measured location of the wheel.

Many of the features described above make the total alignmentmeasurement easier, but not all features are required. At least one podshould be provided per wheel, each pod containing at least one camera toprovide an image of the wheel. A gravity gauge, and at least one angularmeasuring device to measure the angle of the pods with respect to eachother in a plane essentially horizontal, should be provided. The pod canbe attached to the rack with arms that move on a track attached to therack, or the pod can be attached to an arm attached to the rack and apivot point essentially in the middle of the pod which lets the camerastwist and scan horizontally for the wheel.

FIG. 11 is a partial perspective view of this variation. The moduleincludes a single arm 30 to which sensor pod 22 is pivotably mounted atthe end remote from the lift. The arm can be retracted when not in useor extended when in use, in the manner previously described, and held inposition by locking knob 34. The technician may move of the podpivotally or angularly and then lock it in place for the remainder ofthe measurement of that vehicle. Alternatively, such movement can bedone automatically with servos, motors, pistons, or other movingdevices, controlled in a manner as described with respect to FIG. 9.

There may be instances in which attaching the camera module to the sideof the rack is not practical, as there might be air hoses and otherobjects already on the rack that make mounting difficult. In some cases,one might want to move the system from one rack to another and thus notwant a fixed mount to the rack. In these cases, the camera module can beplaced on a stand beside the vehicle. FIG. 12 is a partial perspectiveview of this variation.

Stand assembly 100 includes a base 102 that is positioned about track104, which is parallel to the lift rack, and supported by wheels 106.Bar 24, which holds the pod sensors 22 at a fixed distance, is supportedat a defined height by vertical beam 106. Motors 108 and 110 can becontrolled by the CPU 70 to manipulate the orientation (in yaw andpitch) of the camera module to acquire and track the wheels. Motor 112,affixed to base 102 is engageable with pinion 114 to movably positionthe stand assembly along track 104 through interaction between thepinion and rack 116 affixed thereto. Motor 112 may also be under thecontrol of CPU 70 as described in connection with FIG. 9.

Each module sensor pod of the systems disclosed may contain a gravitygauge 50, as described above with respect to FIG. 7. Once the positionof the wheel is known with respect to the camera, wheel camber can bemeasured with the use of the gravity gauges. As the relative location ofthe camera to the gravity gauge measurement is known by previouscalibration, the relative location of the camera(s) to wheel is all thatis needed to calculate the camber of each wheel.

Calibration of the camber gauge can be performed by placing a wheel inthe field of view of the module camera(s). By use of a gravity gaugeattached to the wheel, the wheel is adjusted to align with a verticalplane (zero camber). By then measuring the angle of the wheel in thecamera(s) and measuring the current reading of the gravity gauge, acorrection factor is obtained. This correction factor is subtracted fromrelative readings of the wheel and camber gauge on subsequentmeasurements to calculate the current camber of the wheel being viewed.

Each module pod contains a set of toe gauges similar to ones found inconventional aligners. There can be a minimum of six total toe gaugesfor the complete system. The use of eight toe gauges can provide a levelof redundancy, useful if one set is blocked. Another set per pod canprovide an inter-pod calibration or a calibration check. The toe gaugesestablish the reference of each pod with respect to each other(angularly) in the toe plane (horizontal plane). The toe gauge maycomprise a linear camera and at least one LED on each pod, or an areacamera and at least one LED, or a laser and sensor, or any ofconventional aligners used in well defined methods. The camera(s) ineach sensor pod obtain an image of the wheel. By finding wheel features,the wheel is located with respect to the pods. From knowledge of the“toe” orientation of each of the pods measured from the toe sensors inthe pods and the relative location of the wheel in the camera's two setsof measurements, and the previously calibrated toe sensor measurement tocamera orientation, the toe (and related measurements; thrust angle.etc.) of the vehicle can be determined.

The relative position of the camera to a toe sensor can be calibrated asfollows. A vehicle with known toe, for example 0.00 degrees toe, isused. The system is used to measure toe with respect to the pods and therelative location of the wheel. The actual reading of toe for the wheelcan be calculated from the measurement of the toe gauges of the pod, therelative position of the wheel to the cameras, and the toe to cameracorrelation. A correction factor is thus established.

Various alignment measurements that can be performed in the presentsystem are described more fully using the following definitions.

A ray R is a straight line in three dimensional space. A ray is definedby a unit vector U in the direction of the ray line, and P, the vectorfrom the origin to a point on the ray line in space. Then vectors fromthe origin to any point on the line in space are given by:R═P+t*Uwhere t is the distance along the line from P. The point on the ray lineP is chosen such that P is normal to U (the dot product of P and U iszero).

Rotating the wheel on its axis defines the wheel axis ray. This is theline in space about which the wheel rotates. The wheel axis vector is aunit vector in the direction of this line. The rim center point is thewhere the wheel axis ray intersects the rim plane. The rim circle liesin the rim plane.

The coordinate systems (CS) contemplated herein are Cartesian coordinatesystems. A Cartesian coordinate system is a three-dimensional coordinatesystem consisting of an origin point and three orthogonal unit vectoraxes. When two coordinate systems are being relatively defined, onecoordinate system is called the primary coordinate system, and the othercoordinate system is called the secondary coordinate system. The unitaxis vectors of a primary coordinate system are called the X, Y and Zaxes. The unit axis vectors of a secondary coordinate system are calledU0, U1 and U2. The origin of the secondary coordinate system is definedby a vector C from the origin of the primary coordinate system.

When one coordinate system is defined relative to another, the center ofthe secondary coordinate system is defined by a vector C from the originof the primary coordinate system, and three unit vectors U. Thecomponents of C, U0, U1 and U2 are defined relative to the primarycoordinate system. The three unit vectors U form a 3×3 matrix. This isthe rotation matrix that rotates the base vectors of the primary CS (X,Y, Z) into the base vectors of the secondary CS (U0, U1, and U2).

To transform a vector V defined in the primary coordinate system (vx,vy, vz) to the secondary CS (v0, v1, v2): 1) Subtract C from V; 2)Multiply the result by the transpose of the matrix U. The resultingvector (v0, v1, v2) will be relative to the origin of the secondarycoordinate system and its three components (v0, v1, v2) will becomponents in the U0, U1, and U2 directions.

To transform a vector V (v0, v1, v2) in the secondary coordinate systemdefined by C and U to the primary CS (vx, vy, vz): 1) Multiply V by thematrix U; 2) add C. The resulting vector will be relative to the originof the primary coordinate system and its three components (vx, vy, vz)will be components in the primary coordinate system X, Y, and Zdirections. In the coordinate system of a camera (CCS), Z is a directionout from the camera towards the vehicle; X and Y are the pixelcoordinate directions (Y down).

A wheel (or target) coordinate system is imbedded in the wheel. Lookingat the wheel, X is to the right; Y is up; and Z is the outward normal.If a target is normal to a wheel, their Z directions are opposite. Awheel's coordinate system may also be defined as a rim plane coordinatesystem. A wheel of an automotive vehicle consists of a tire, a wheel rimstructure which holds the tire, and a suspension system which connectsthe rim structure to the vehicle. Looked at from the side, the circlewhere the wheel rim ends and the tire sidewall begins is called the rimcircle. This circle lies in a plane called the rim plane. This rim planedefines a rim coordinate system. The origin of this coordinate system isthe center of the rim circle. The outward normal to the rim plane is theZ axis of the rim coordinate system. The X and Y axes are any two unitvectors lying in the rim plane, normal to the Z axis and each other.

The vehicle coordinate system is imbedded in the vehicle. The centerpoints of the wheel lie in the XY plane of the vehicle coordinatesystem. Z points up. Y is the Geometric Center Line (GCL) drawn from themidpoint of the rear rim centers to the midpoint of the front rimcenters. X is the Geometric Center Line Perpendicular (GCLP). The Z axisof the vehicle coordinate system is defined as the upward direction ofthe gravity vector, or the upward normal of the lift rack plane.

From calibration, the relative orientations of the camera coordinatesystems and the Z axis of the vehicle coordinate system are known. The Xand Y axes of the vehicle coordinate system may be rotated about the Zaxis of the vehicle coordinate system without changing the Z axis of thevehicle coordinate system. A method of defining their specificdirections is defined as follows: For each wheel, get the wheel planecoordinate system (target plane or rim plane) in the CCS. Get the centerpoint between the two front wheel planes and the center point betweenthe two rear wheel planes in the camera coordinate system. Define the Yaxis of the vehicle coordinate system as the unit vector from the rearcenter point to the front center point. The X axis of the vehiclecoordinate system (GCLP) is then the vector cross product of the Y and Zunit vectors. The VCS origin is defined as the midpoints of the frontand rear center points. The vehicle dimensions (front and rear left toright widths, and front to rear length) are known for the specificvehicle. The wheel axes are defined as either unit vector about whichthe wheel rotates. This axis is measured as the axis about which atarget affixed to the wheel rotates, or is defined as the normal to therim plane.

Alignment parameters may be determined from interaction among thevarious coordinate systems described above. For all four wheels, toeline and camber may be computed in the following manner. Individual toeline unit vectors are defined by projecting the wheel axis vectors ontothe vehicle coordinate system XY plane and rotating by 90°, clockwise onthe left side, counterclockwise on the right. The x component of the toeline vector is the vehicle coordinate system Y (vehicle forward)component of the wheel axis vector, negated for the right side wheels.The Y component of the toe line vector is the absolute value of thevehicle coordinate system X (vehicle side) component of the wheel axisvector. The Z component of the toe line vector is set to zero, and thetoe line vector is then normalized. With this definition, the toe angle,measured positive from toe line X towards toe line Y, increases fromzero as the wheel axis vector component in the vehicle forward directionincreases.

Camber is the inward or outward tilt angle of the wheel off of thevertical direction (vehicle coordinate system Z), positive if the top ofthe wheel tilts outward. It is thus computed as the angle whose sine isthe Z component of the wheel axis unit vector in the vehicle coordinatesystem.

For the two front wheels, caster is the forward or rearward tilt angleof the steering axis off of the vertical direction (vehicle coordinatesystem Z), positive if the tilt is rearward at the top (clockwise offvertical as viewed from the left side of the vehicle). Caster is thuscomputed as the angle (in degrees) between the projection of thesteering axis on the vehicle coordinate system YZ plane and vehiclecoordinate system Z.

SAI (Steering Axis Inclination) is the inward or outward tilt angle ofthe steering axis off of the vertical direction (vehicle coordinatesystem Z), positive if the tilt is inward, clockwise off vertical on theleft, counterclockwise on the right, as viewed looking forward (vehiclecoordinate system Y). It is thus the angle of the steering axis on thevehicle coordinate system XZ plane and vehicle coordinate system Z,negated for the right side. Thrust Line is the unit vector bisecting therear individual toe line unit vectors. Thrust Angle is the angle betweenthe thrust line and GCL (vehicle coordinate system Y). It is computed asthe angle whose tangent is the ratio of the vehicle coordinate system Xand Y components of the thrust line unit vector.

Front toe angle is the angle between the thrust line and the individualfront toe line. “Toe in” is positive, so the left front toe angle ispositive clockwise off the thrust line, and the right front toe angle ispositive counter-clockwise. Toe angle is computed as the angle betweenthe toe line and thrust line unit vectors, multiplied by the sign of thedifference between the vehicle coordinate system X components of the toeline and thrust line. Rear toe is the same as front, except that thethrust line is replaced by the GCL (vehicle coordinate system Y). Totalfront or rear toe is the angle between the individual toe lines,computed as the sum of the toe angles.

Setback is the angle between the front or rear axis unit vector(normalized vector between rim center points in the vehicle coordinatesystem) and the GCLP (vehicle coordinate system X), positive when theright wheel is “set back” from the left wheel. It is computed as theangle whose tangent is the ratio of the vehicle coordinate system X andY components of the front or rear axis unit vector.

Steering angle is the angle in degrees between the bisectors of thefront and rear individual toe lines, i.e. the angle between the bisectorof the front individual toe line and the thrust line, which is thebisector of the rear individual toe line. It is computed as the anglebetween the line bisecting the front toe line unit vectors and thethrust line unit vector. For each front wheel, the steering axis in thevehicle coordinate system is computed as the ray about which the wheeltarget plane rotated from the right to the left caster swing position,always defined to point upward (positive vehicle coordinate system Zcomponent).

To acquire the data necessary to compute the parameters of interest, theposition of wheels relative to a vehicle coordinate system (VCS) must bedetermined. Thus, the system acquires the position of each wheelrelative to one or more cameras. The relative positions of the cameraswith respect to each other is known by calibration methods to bedescribed hereinafter. Also, the positions of the cameras relative tothe VCS is determined by calibration methods to be described. Onecamera, or a group of cameras, is dedicated to each wheel to make themeasurements for that wheel. Such a group of cameras, even if there isonly one camera in the group, is called a pod. The relative positions ofthe cameras in such a pod is determined by a calibration procedure to bedescribed. Additional cameras, or other calibration procedures to bedescribed, are used to establish the relative positions of these podswith respect to each other, so that the relative position of any podwith respect to the VCS may be defined.

A process of measuring a target coordinate system relative to a cameracoordinate system has been described in the commonly assigned patentspreviously identified herein. This process requires a target assembly tobe attached to the wheel and imaged by one or more cameras. This targetassembly consists of a plurality of visible markers. The spatialrelationship of these markers to each other is well known and accuratelydefined for a particular target. The relative positions of the camerasis known by calibration procedures described in the referenced patents.

Alternative embodiments allow one or more cameras to image the wheel andacquire equivalent information, in particular, the position of the rimplane relative to the cameras' coordinate system (CCS). Three suchmeasurement embodiments are described more fully below. The firstembodiment requires no additions or modifications to the wheel. In thisembodiment, a camera or cameras image the wheel as is. The position ofthe rim plane relative to the camera coordinate system is measured by aprocess to be described. In the second embodiment, a plurality ofvisible markers are individually affixed to the surface of the wheel orwheel rim and imaged by a plurality of cameras. The spatial relationshipof these markers to each other is not known or pre-determined, but, onceaffixed, these markers remain fixed relative to each other during themeasurement process. Their positions relative to the CCS are measured bya process to be described. Since these markers and the rim plane form arigid body, the position of the rim plane relative to the cameras'coordinate system can be determined by a calculation to be described. Inthe third embodiment, a pattern of light is projected onto the wheel andimaged by a plurality of cameras. The position of the projector relativeto the cameras can be made known by a calibration procedure to bedescribed. The position of points on this pattern on the wheel surfaceis determined by a method to be described. This collection of threedimensional points defines the surface of a three dimensional objectthat can be identified with the known surface shape of a wheel. Theposition of this shape defines the position of the rim plane relative tothe camera CCS.

In the first measurement embodiment, there are no additions ormodifications to the wheel, and one or more cameras image the wheel asis. The position of the rim plane relative to the camera CCS is measuredby the following process. The outward normal to the rim plane is the U2axis of the rim coordinate system. The U0 and U1 axes are any two unitvectors lying in the rim plane, normal to the U2 axis and each other.Due to the symmetry of a circle, only the center and the U2 axis need beuniquely defined. The U0 and U1 axes can be rotated about the normal byan arbitrary angle without changing the rim circle center or normal,unless an additional feature in the plane can be identified to definethe orientation of these two vectors. This rim coordinate system (rimCS) is taken as the secondary coordinate system, and the camera CCS istaken as the primary coordinate system. The focal point of the camera isthe origin of the CCS, and the directions of the camera's rows andcolumns of pixels define the X and Y axes, respectively. The cameraimage plane is normal to the Z axis, at a distance from the origincalled the focal length. Since the rim circle now lies in the rim plane,the only additional parameter needed to define the rim circle is itsradius.

For any position and orientation of the rim CS relative to a CCS in thatcamera's field of view, the rim circle projects to a curve on the cameraimage plane. Edge detection means well known in the optical imagingfield are employed to find points in the camera image plane lying onthis curve. The following method may be used to find and measure pointson this curve to sub-pixel accuracy.

The sidewall portion of the tire is adjacent to, and radially outwardfrom the rim circle. Such sidewalls typically have different opticalproperties than the rim material. Thus the intensities of the pixels inthe sidewall segments of the image differ from the intensities of thepixels in the rim segments of the image. A plurality of closely spacedline segments of pixels, each crossing the sidewall-rim interface atapproximately right angles to the rim edge curve in the image, isdefined in the image data processing system. These lines span the wholeclosed curve of the rim edge in the image. For each such line of pixels,a subset consisting of a fixed number of contiguous pixels is defined.This defined subset is moved along the line of pixels until thefollowing conditions are met: all pixels in a contiguous group at oneend of the segment are identified as sidewall; all pixels in acontiguous group at the opposite end of the segment are identified asrim. The following definitions are set forth for making thesedeterminations.

-   -   Is=Average intensity of the sidewall pixels.    -   Ir=Average intensity of the rim pixels.        Threshold(T)=(Is+Ir)/2.    -   I1 and I2=Intensities of two contiguous pixels such that: I1<=T        and I2>=T.        F=(T−I1)/(I2−I1).

Then the sub-pixel position of the edge point along this line of pixelsis: F+Integer position of pixel with intensity I1. Given this set ofedge points in the camera image plane (XY plane of the CCS), therelative position of the rim CS is determined as follows:

-   -   Let the rim CS be defined relative to the CCS by:        -   C=Vector to rim CS center.        -   U2=Normal axis of rim CS.        -   rr=Radius of rim circle in rim plane CS.            Then any vector from the origin of the CCS to a point in the            rim plane is given by: R=C+Q, where Q is a vector lying in            the rim plane. Since Q is in the rim plane, it is normal to            U2.

The origin of the CCS is the focal point of the camera. The camera imageplane is normal to the Z axis of the camera and intersects the CCS Zaxis at a distance f from the CCS origin, where f is the focal length ofthe camera. Then:

-   -   xi,yi,f=CCS coordinates of the ith rim edge point in the CCS.        Let V be the vector from the CCS origin to this point on the        camera image plane. The components of V in the CCS are then        (xi,y1,f). Extend this vector to the rim plane:    -   k1*V=Extended vector from origin of CCS to point on rim plane.        Since this point lies in the rim plane, k1*V=C+Q. Since Q is        normal to U2:        k1*V*U2=C*U2,        k1=(C*U2)/(V*U2),        Q=k1*V−C.        Let:        UQ=Q/|Q|,        Qrr=rr*UQ.

Qrr is a vector in the rim plane, from the rim plane origin, parallel toQ, but whose length is rr. So Qrr lies on the rim circle, and is theclosest point to Q in the rim plane.

C+Qrr=Vector from the CCS origin to this point.

V2 is the vector from the CCS origin, parallel to C+Qrr, that intersectsthe camera image plane. V2=k2*(C+Qrr)=(x,y,f).

Take the Z component:V2z=k2*(Cz+Qrrz)=f.So:k2=f/(Cz+Qrrz),x=k2*(Cx+Qrrx),y=k2*(Cy+Qrry).

On the camera image plane, (x,y) is the projection of the point on therim circle closest to the measured edge point (xi,yi). (x,y) is afunction of (xi,yi) and the vectors defining the rim plane CS: C and U2.

The measured point (xi,yi) should have been the projection onto thecamera image plane of a point on the rim circle, so the differencebetween (xi, yi) and the corresponding (x,y) as defined above, on thecamera image plane, is a measure of the “goodness of fit” of the rimparameters (C and U2) to the measurements. Summing the squares of thesedifferences over all measured points gives a goodness-of-fit value:Φ=Σ((xi−x)2+(yi−y)2) i=1, . . . , N,where N is the number of measured points. A “least-squares fit”procedure, well known in the art, is used to adjust C and U2, thedefining parameters of the rim circle, to minimize Φ, given the measureddata set {xi,yi} and the rim circle radius rr.

In a variation of this embodiment, two or more cameras whose relativepositions are known by a calibration procedure can image the wheel andrim and the data sets from these two cameras can be used in the abovecalculation. In this case:Φ=Φ0+Φ1+ . . . +Φn,where Φo is defined above for camera 0, and Φ1 thru Φn are similarlydefined for the other cameras, with the following difference: the rimplane C and U2 used for the other cameras are transformed from the CCSof the first camera into the CCS of the other camera. The CCSs of theother cameras are defined (by a calibration procedure) relative to theCCS of the first camera.

The rim plane and circle have now been determined based on multiple setsof curve point data, comprised of sets of measured points, in cameraimage planes, and thus spatial characteristics of the rim plane andcircle are now known. As the rim plane and circle are part of the wheelassembly (including wheel rim and tire), spatial characteristics of thewheel assembly can be determined based on the spatial characteristics ofthe rim plane and circle.

In the second measurement embodiment, a plurality of visible markers areindividually affixed to the surface of the wheel or wheel rim and imagedby a plurality of cameras. The spatial relation of these markers to eachother is not known or pre-determined, but, once affixed, these markersremain fixed relative to each other during the measurement process.Their positions relative to the camera CS are measured. Since thesemarkers and the rim plane form a rigid body, the position of the rimplane relative to the CCS can be determined by calculation. Thismeasurement method is more fully described, for example, in U.S. Pat.No. 5,724,129 to Matteucci.

In the third measurement embodiment, a pattern of light is projectedonto the wheel and imaged by a plurality of cameras. The position of theprojector relative to the cameras is known by a calibration procedure.The position of points on this pattern on the wheel surface isdetermined. This collection of three-dimensional points defines thesurface of a three-dimensional object that can be identified with theknown surface shape of a wheel. The position of this shape relative tothe VCS defines the position of the wheel plane relative to the camerasCCS. This measurement method is more fully described in U.S. Pat. Nos.4,745,469 and 4,899,218, both to Waldecker.

Various techniques may be used to derive the determinations made in thethree measurement embodiments discussed above. To acquire the datanecessary to compute the parameters of interest, the position of wheelsrelative to a vehicle coordinate system (VCS) must be determined. To dothis, two relative positions must be known: 1) The position of the wheelCS relative to the camera CS(CCS), and 2) the position of the CCSrelative to the VCS. Given this information, the positions of the wheelsrelative to the VCS can be defined by the following calculation.

Given a primary (such as the vehicle) CS, and an intermediate (such asthe camera) CS defined relative to the primary CS, and an object (suchas the rim) CS defined relative to the camera CS, the position of therim CS relative to the vehicle CS is determined as follows: In thevehicle CS, with axes X,Y and Z and origin at (0,0,0), the camera CS isdefined by vector CC from the origin of the VCS to the origin of theCCS, and axes UC0, UC1 and UC2. Elements are defined with respect to theVCS as follows:CC=CCx*X+CCy*Y+CCz*ZUC0=UC0x*X+UC0y*Y+UCI0z*ZUC1=UC1x*X+UC1y*y+UC1z*ZUC2=UC2x*X+UC2y*Y+UC2z*ZMatrix UC is defined as:

-   -   |UC0 x UC1 x UC2 x|    -   |UC0 y UC1 y UC2 y|    -   |UC0 z UC1 z UC2 z|

In the CCS, with axes UC0, UC1 and UC2 and origin at CC, the RCS isdefined by vector CR from the origin of the CCS to the origin of theRCS, and axes UR0, UR1 and UR2. Elements are defined with respect to theCCS as follows:CR=CR0*UC0+CR1*UC1+CR2*UC2UR0=UR00*UC0+UR01*UC1+UR02*UC2UR1=UR10*UC0+UR11*UC1+UR12*UC2UR2=UR20*UC0+UR21*UC1+UR22*UC2Matrix UR is defined as:

-   -   |UR00 UR10 UR20|    -   |UR01 UR11 UR21|    -   |UR02 UR12 UR22|

Given the CCS so defined relative to VCS, and the RCS so definedrelative to the CCS, standard matrix algebra gives the RCS definedrelative to VCS by:URinVCS=UC*URCRinVCS=(UC*CR)+CC

The RCS relative to the CCS, i.e. the position of a wheel relative to acamera, is measured by the system by one of the exemplary methods of theembodiments described above. Thus, to determine the position of thatwheel (RCS) relative to the VCS, as required for the vehicle parametermeasurements described above, it is necessary to know the position ofthe camera (CCS) relative to the VCS.

If multiple cameras are used to measure the same object, such as awheel, the measurements of the additional cameras can be expressed asmeasurements in the first camera's CS by the same mathematical relationsas above, with the first camera CS in place of the VCS, and theadditional camera CS in place of the camera CS, in the abovemathematical expressions. These positional relations between cameras arecalled Relative Camera Position, or RCP.

The relative position of a camera with respect to the VCS, and therelative positions of cameras with respect to each other (RCP) aredetermined by the following calibration techniques.

One camera, or a group of cameras, is dedicated to each wheel to makethe measurements for that wheel. Such a group of cameras, even if thereis only one camera in the group, as noted earlier, is called a pod. Therelative positions of the cameras in the pod are fixed and permanentwith respect to each other. To determine these fixed intrapod RCPs, onecamera in the pod is defined to be the primary camera, and the RCP ofeach other camera is determined with respect to the primary camera bypositioning a target so that part or all of the target is simultaneouslyvisible in the field of view of both cameras.

Alternatively, the measurement pods may commonly view a multifacetedsolid with known unique markings on each face. The positionalrelationships between markings on each face of the solid arepredetermined and stored in the computer. Since the relative positionalrelationships between the markings on each face of solid arepredetermined and known, and the relative positions of each pod withrespect to the solid are know by measurement, the relative positions ofthe measurement pods with respect to each other can be determined.

Alternatively, a target is placed in the field of view of one camera ofthe pair, then moved by a known amount to a position in the field ofview of the second camera. Since the movement is known, the measuredposition of the target as seen by one camera can be defined in the CS ofthe other camera even if, in this physical position, the target is notin the field of view of the other camera.

Other common types of objects with known geometrical characteristics canbe used for performing the calibration process, such as, for example, areference platform with known grid lines. Other approaches that can beused to determine the relative positions between the measurement podsand cameras are described in commonly assigned U.S. Pat. No. 5,809,658to Jackson et al., entitled “Method and Apparatus for CalibratingAlignment cameras Used in the Alignment of Motor Vehicle Wheels,” issuedto Jackson et al. on Sep. 22, 1998; and in commonly assigned U.S. Pat.No. 6,968,28, to Jackson et al., entitled “Self-calibrating,multi-camera machine vision measuring system.”

In any case, the “same” target position (TCS) is known in the CS of bothcameras. Given the TCS defined relative to CCS1 (the CS of the primarycamera), and the TCS defined relative to CCS2 (the CS of the othercamera), CCS2 defined relative to CCS1 is defined as follows:

Let CT1 and UT1 be the center vector and matrix of the target CS inCCS1, and CT2 and UT2 be the center vector and matrix of the target CSin CCS2, and UT2T be the transpose of UT2.U2inCCS1=UT1*UT2TC2inCCS1=(U2inCCS1*CT2)+CT1C2inCCS1 is the vector from the origin of CCS1 to the origin of CCS2,defined in CCS1. U2inCCS1 is the matrix of axis unit vectors CCS2,defined in CCS1.

This intra-pod calibration defines the RCP of any camera in a pod withrespect to the primary pod camera. Thus all measurements by any camerain the pod can be related to measurements of the primary pod camera.

Inter-pod calibration defines the RCP between the primary camera in onepod and the primary camera in another pod, such as between pods viewingdifferent wheels. Calibration techniques differ, depending upon whetherthe relative position of the two pods is fixed and permanent, referredto herein as fixed pod case, or the relative position of the two podscan vary from measurements of one vehicle to another vehicle, referredto herein as non-fixed pod case.

In the fixed pod case, inter-pod RCP need only be measured once, by thesame methods as described above for intra-pod RCP. In the non-fixed podcase, inter-pod RCP must be determined anew for each vehiclemeasurement. For this case, alternative techniques may be applied.

In a first inter-pod RCP determination technique, one pod includes acamera that views a target incorporated in the second pod. The RCP ofthis calibration camera to the primary camera in the same pod isdetermined by the methods described above. The position of the targetrigidly attached to the second pod relative to the cameras in that pod,called Relative Target to Camera Position, or RTCP, is determined asfollows. A camera external to the pod views both the target attached tothe pod, and another target, also external to the pod. Using themathematical methods described above, the relative positions of theexternal target and the target attached to the pod, called the RelativeTarget to Target Position, or RTTP, is determined.

This external target is also in the field of view of a pod camera, sothe position of the external target relative to the pod camera ismeasured. The external target is used as the intermediate CS asdescribed above, and the position of the target attached to the pod isthus calculated relative to the pod camera. Given the relative positionof the calibration camera to the camera(s) in the first pod, therelative position of the target attached to the second pod as measuredby the calibration camera in the first pod, and the relative position ofthat attached target to the camera(s) in the second pod, the RCP of thecamera(s) in the second pod is determined by the mathematical methodsdescribed above.

The above calibration gives the relative camera positions (RCPs) of allcameras in the system with respect to one camera. This is equivalent tohaving one big camera CS with respect to which all the wheel rim CSs aremeasured. The VCS can then be defined relative to this CS, and viceversa, as follows. The center points of the wheel rim CSs are allmeasured and hence known in the CCS. These rim CS center points, knownrelative to the CCS, lie in the XY plane of the VCS. The normal to thatplane is the Z axis of the VCS. The VCS Y axis, which is the GCL(Geometric Center Line) of the vehicle, is defined in the CCS by theline drawn from the midpoint of the rear rim centers to the midpoint ofthe front rim centers. The VCS X axis, which is the GCLP (GeometricCenter Line Perpendicular) of the vehicle, is the vector cross productof the VCS Y axis and the VCS Z axis. The direction of the VCS X axis isfrom the left toward the right side of the vehicle, so the VCS Z axispoints upward. The VCS origin is defined as the midpoints of the frontand rear center points. The VCS relative to the CCS is thus defined, asrequired.

The VCS Z axis may be defined to be the upward direction of gravity.This direction is determined by use of a gravity gauge. The gravitygauge outputs a measurement, readable by the electronic data processingpart of the system, indicating the orientation of the body of thegravity gauge with respect to the direction of gravity. To calibrate theorientation of a camera relative to the direction of gravity, thegravity gauge and camera are both rigidly attached to a structure suchas a camera pod. One or more pod cameras image a target whoseorientation with respect to the direction of gravity is known. Thetarget may incorporate a linear structure visible to the camera(s) whichis free to move under the influence of gravity to a position at whichthe linear structure aligns with the direction of gravity. The directionof this line, measured in the CCS from the camera images, gives thedirection of gravity in the CCS. The simultaneous reading of the gravitygauge gives the relation between the gravity gauge data and theorientation of the CCS with respect to gravity, and vice versa.

Alternatively, the VCS Z axis may be defined to be the upward normal tothe plane on which the vehicle is supported. This normal direction isdetermined by imaging three or more points on the supporting planesimultaneously by two or more cameras with known RCP. Triangulation thendefines the position of these points, and hence the plane in which theylie, with respect to the CCS. The normal to this plane, which may bechosen as the VCS Z axis, is thus known relative to the CCS.

In yet another variation, a target in the field of view of one or morecameras (whose RCP is known) is attached to a support structure, such asa tripod, which stands and is free to move on the plane supporting thevehicle. Such a supported target may be rotated about an axis normal tothe vehicle support plane. The camera measures the target plane relativeto the CCS in two or more of these rotated positions, and the axis ofrotation, which is the normal to the vehicle support plane, and may bechosen as the VCS Z axis, is thus known relative to the CCS.

For any of these definitions of the VCS Z axis, the VCS Y axis (the GCL)is defined as above, and the VCS X axis is then defined, as describedabove, by the vector cross product of the VCS Y axis and the VCS Z axis.Given the VCS so defined and measured, the vehicle alignment parametersare then derived as described earlier.

In a second inter-pod RCP determination technique, the relative positionof each pod CS with respect to the VCS is determined directly.Additional gauges may be incorporated in the pods, such as gravity andtoe gauges. The RCPs of the cameras within a pod are still known, asdescribed above. The use of the gravity gauges to define the VCS Z axiswith respect to the pod cameras has been described above, as has thedefinition of toe.

A toe gauge may comprise a light source in one pod, and a linear lightdetecting array rigidly attached to a focal slit collimator in anotherpod. The orientation of this linear array and slit assembly is such thatthe line of the slit is in the direction of gravity, and the line of thedetector array is normal to gravity. Light from the source in one podpasses through the slit in the other pod and illuminates one location onthe linear detector array. Such array and slit assemblies are commonlyavailable and are internally calibrated as part of their manufacturingprocess so the following parameters are known: F: the normal distancefrom the slit to the array; D0: the position of the point on the line ofthe detector array such that the line from that point to the slit isnormal to the line of the detector array.

When light from the source in the other pod passes through the slit itproduces a narrow illuminated region on the detector array. Electronicsthat are part of the array determine: D: The position of the point onthe line of the detector array at the center of illuminated region.Then:tan (A)=(D−D0)/Fwhere A is the angle of the light beam, and hence a straight line in theVCS XY plane, from the other pod with respect to the normal to the lineof the array.

Two such detector assemblies, and two light sources, are incorporated ineach of the four pods associated with and adjacent to each wheel of thevehicle. The angles of each of the four sides of the vehicle aremeasured by pairs of detector assemblies located on corresponding pods.Only three of these measurement pairs are required to obtain all of thedesired wheel alignment information related to the vehicle beingmeasured. The additional sensor pair provides redundant information thatcan be used to check the calibration of the system. Alternatively, thealignment system can be configured using only three sensor pairs. Anglesmeasured by the detector assemblies are in the VCS XY plane. Theposition of the rim plane of each wheel is determined relative to thecorresponding pod cameras in the CCS as previously described. Therelative position of each pod to its corresponding detector assembly isknow by prior calibration. Therefore, the relative position of the rimplane to the VCS can be determined. Alternatively, the position of therim plane with respect to the detector array can be determined by themeasured rim plane position relative to the corresponding pod and theknown relationship of the pod to the corresponding detector array. Thisrelationship can be used with the angles measured by the detector arraysto compute the vehicle wheel alignment information as described indetail in U.S. Pat. No. 5,519,488 to Dale.

It is to be understood that the present invention is capable of use invarious other combinations and environments and is capable of changes ormodifications within the scope of the inventive concept as expressedherein. For example, the fixed distance between the pair of sensors of amodule need not be set at the diameter of a vehicle wheel, so long aseach of the sensors has sufficient field of view of the wheelcharacteristic image. Individual protective covers for respectivesensing modules may be spaced to avoid relocation of a module to bestowed. The concept of retracting the module when not in use isapplicable to other stowed positions, for example, without longitudinaldisplacement. Although each module has been shown to comprise twocameras to obtain high accuracy, a single camera may be employed in eachmodule. Fewer gravity sensors can be used in cases where the location ofa camera is known by the system with respect to another camera. Inanother variant, a single axis gravity sensor may be predominantlymounted perpendicular to the rack centerline, thus measuring the gravitygenerally in the camber plane of the vehicle. In this variation, onlyone of the camera modules would need a gravity gauge.

1. A vehicle wheel alignment sensing system for use with a verticallyadjustable vehicle supporting lift, the system comprising: an opticalsensing mechanism operably coupled to the supporting lift for movementin unison with the lift, the sensing mechanism comprising at least onesensing module having a field of view that encompasses a wheel of avehicle positioned on the lift.
 2. An alignment sensing system asrecited in claim 1, wherein the sensing module comprises a pair ofsensors positioned at a known orientation with respect to each other. 3.An alignment sensing system as recited in claim 2, wherein the pair ofsensors are positioned at a fixed distance from each other.
 4. Analignment sensing system as recited in claim 2, wherein the sensorscomprise cameras.
 5. An alignment sensing system as recited in claim 4,further comprising a plurality of said sensing modules coupled to thesupporting lift for viewing respective wheels of the vehicle.
 6. Analignment sensing system as recited in claim 5, wherein the fixeddistance is set to place the pair of sensors along a longitudinaldirection of the lift proximate opposite ends of the wheel diameter,whereby the field of view of each sensor encompasses a substantialportion of the wheel.
 7. An alignment sensing system as recited in claim6, further comprising a mounting member attachable to one side of thelift, the mounting member comprising a plurality of positions forsecurely mounting a pair of said sensing modules.
 8. An alignmentsensing system as recited in claim 7, wherein a mounting member positionfor at least one of the pair of sensing modules is selectable inaccordance with the wheel base of the vehicle.
 9. An alignment sensingsystem as recited in claim 7, further comprising a second mountingmember attachable to the opposite side of the lift, the mounting membercomprising a plurality of positions for securely mounting a pair of saidsensing modules.
 10. An alignment sensing system as recited in claim 2,further comprising a deployment mechanism coupled between the lift andthe sensing module, said deployment mechanism having an extendedposition in which the sensing module extends outwardly from the lift forviewing the wheel during an alignment procedure and a retracted positionadjacent the lift for stowing.
 11. An alignment sensing system asrecited in claim 10, wherein the retracted position is displaced in thelongitudinal direction from the extended position.
 12. An alignmentsensing system as recited in claim 10, further including a protectivecover attachable to the lift to extend above sensing module in theretracted position.
 13. An alignment sensing system as recited in claim10, comprising means for locking the deployment mechanism in each of theextended position and retracted position for stabilizing the sensingmodule.
 14. An alignment sensing system as recited in claim 7, furthercomprising a deployment mechanism coupled between the lift and eachsensing module, said deployment mechanism having an extended position inwhich the sensing module extends outwardly from the lift for viewing thewheel during an alignment procedure and a retracted position adjacentthe lift for stowing.
 15. An alignment sensing system as recited inclaim 14, wherein the retracted position is displaced in thelongitudinal direction from the extended position.
 16. An alignmentsensing system as recited in claim 15, further including a protectivecover attachable to the lift to extend above the sensing module in theretracted position.
 17. An alignment sensing system as recited in claim15, wherein a mounting member position for at least one of the pair ofsensing modules selectable to position the pair of sensing modulesadjacent to each other in their retracted positions; and the protectivecover is configured to extend over both sensing modules in theirretracted positions.